1. Field of the Invention
Embodiments of the present invention relate to analyte sensors. In particular, certain embodiments of the present invention relate to methods of using non-consuming intravascular glucose sensors to achieve tight glycemic control.
2. Description of the Related Art
A specific type of polyneuropathy develops in patients that are treated within an intensive care unit (hereinafter also designated ICU) for several days to weeks and this for a variety of primary injuries or illnesses. This polyneuropathy, known as “Critical Illness Polyneuropathy” (hereinafter also designated CIPNP) occurs in about 70% of patients who have the systemic inflammatory response syndrome (SIRS) (Zochodne D W et al. 1987 Polyneuropathy associated with critical illness: a complication of sepsis and multiple organ failure. Brain, 110: 819-842); (Leijten F S S & De Weerdt A W 1994 Critical illness polyneuropathy: a review of the literature, definition and pathophysiology. Clinical Neurology and Neurosurgery, 96: 10-19). However, clinical signs are often absent and it remains an occult problem in many ICUs worldwide. Nonetheless, it is an important clinical entity as it (is) a frequent cause of difficulty to wean patients from the ventilator and it leads to problems with rehabilitation after the acute illness has been treated and cured.
When CIPNP is severe enough, it causes limb weakness and reduced tendon reflexes. Sensory impairment follows but is difficult to test in ICU patients. Electro-physiological examination (EMG) is necessary to establish the diagnosis (Bolton C F. 1999 Acute Weakness. In: Oxford Textbook of Critical Care; Eds. Webb A R, Shapiro M J, Singer M, Suter P M; Oxford Medical Publications, Oxford UK; pp. 490-495). This examination will reveal a primary axonal degeneration of first motor and then sensory fibers. Phrenic nerves are often involved. Acute and chronic denervation has been confirmed in muscle biopsies of this condition. If the underlying condition (sepsis or SIRS) can be successfully treated, recovery from and/or prevention of the CIPNP can be expected. This will occur in a matter of weeks in mild cases and in months in more severe cases. In other words, the presence of CIPNP can delay the weaning and rehabilitation for weeks or months.
The pathophysiology of this type of neuropathy remains unknown (Bolton C F 1996 Sepsis and the systemic inflammatory response syndrome: neuromuscular manifestations. Crit Care Med. 24: 1408-1416). It has been speculated to be directly related to sepsis and its mediators. Indeed, cytokines released in sepsis have histamine-like properties which may increase microvascular permeability. The resulting endoneural edema could induce hypoxia, resulting in severe energy deficits and hereby primary axonal degeneration. Alternatively, it has been suggested that cytokines may have a direct cytotoxic effect on the neurons. Contributing factors to disturbed microcirculation are the use of neuromuscular blocking agents and steroids. Moreover, a role for aminoglucosides in inducing toxicity and CIPNP has been suggested. However, there is still no statistical proof for any of these mechanisms in being a true causal factor in the pathogenesis of CIPNP.
Although polyneuropathy of critical illness was first described in by three different investigators, one Canadian, one American, and one French, to date there is no effective treatment to prevent or stop Critical Illness Polyneuropathy.
To date the current standard of practice of care, especially of critically ill patients, was that within the settings of good clinical ICU practice, blood glucose levels are allowed to increase as high as to 250 mg/dL or there above. The reason for this permissive attitude is the thought that high levels of blood glucose are part of the adaptive stress responses, and thus do not require treatment unless extremely elevated (Mizock B A. Am J Med 1995; 98: 75-84). Also, relative hypoglycaemia during stress is thought to be potentially deleterious for the immune system and for healing (Mizock B A. Am J Med 1995; 98: 75-84).
Van Den Berghe, U.S. Patent Publication No. 2002/0107178 A1, disclosed that critical illness in a patient and/or CIPNP can be prevented, treated or cured, at least to a certain extent, by strictly controlling glucose metabolism during said critical illness by applying intensive treatment with a blood glucose regulator, for example, insulin treatment, with clamping of blood glucose levels within a range where the lower limit can be selected to be about 60, about 70 or about 80 mg/dL and the upper limit can be selected to be about 110, about 120 or about 130 mg/dL, more specifically to the normal range (i.e., from about 80 to about 110 mg/dL). The skilled art worker, for example, the physician, will be able to decide exactly which upper and lower limits to use. Alternatively, the range is from about 60 to about 130, preferably, from about 70 to about 120, more preferred, from about 80 to about 110 mg/dL.
Unfortunately, despite the benefits of tight glycemic control in the ICU patient, it has been difficult to implement in part because there are no accurate, real-time, indwelling glucose sensors available. Consequently, it has been a significant burden on the patients and the ICU staff to perform frequent blood sampling for conventional ex vivo blood glucose monitoring.
There has been an on-going effort over many years to use equilibrium chemistry to measure polyhydroxyl compound (e.g., glucose) concentration in bodily fluids. For example, several attempts have been made to detect glucose by fluorescence using dyes associated with boronic acid groups. Boronate moieties bind glucose reversibly. When boronic acid functionalized fluorescent dyes bind glucose, the properties of the dye are affected, such that a signal related to the concentration of glucose may be generated and detected.
Russell (U.S. Pat. Nos. 5,137,833 and 5,512,246) used a boronic acid functionalized dye that bound glucose and generated a signal related to the glucose concentration. James et al. (U.S. Pat. No. 5,503,770) employed a similar principle, but combined a fluorescent dye, an amine quenching functionality, and boronic acid in a single complex. The fluorescence emission from the complex varied with the amount of glucose binding. Van Antwerp et al. (U.S. Pat. Nos. 6,002,954 and 6,011,984) combined features of the previously cited references and also disclosed a device purported to be implantable. A. E. Colvin, Jr. (U.S. Pat. No. 6,304,766) also disclosed optical-based sensing devices for in situ sensing in humans that utilize boronate-functionalized dyes. But despite the effort, no practical intravascular system has been developed and commercialized for in vivo monitoring.
Certain measurable parameters using blood or bodily fluid, such as pH and concentrations of O2, CO2, Na+, K+, and polyhydroxyl compounds, like glucose, have been determined in vivo. The ability to do these measurements in vivo is important because it is necessary to make frequent determinations of such analytes when monitoring a patient. In many instances, a sensor will be analyte specific and therefore a plurality of sensors may be needed to measure several analytes, which can cause attendant discomfort to the patient and add complexity to the electronic monitoring equipment.
In an effort to solve the design problems posed by the limitation in physical dimension for in vivo monitoring, others have incorporated different dyes into one device to get simultaneous readings of two parameters. For example, Alder et al. (U.S. Pat. No. 5,922,612) disclosed a method for optical determination of pH and ionic strength of an aqueous sample using two different dyes on one sensor. Gray et al. (U.S. Pat. No. 5,176,882) taught the use of a fiber optic device incorporating a hydrophilic polymer with immobilized pH sensitive dye and potassium or calcium sensitive fluorescent dyes to measure the analyte concentration in conjunction with pH. In U.S. Pat. No. 4,785,814, Kane also disclosed the use of two dyes embedded in a composite membrane for the simultaneous measurements of pH and oxygen content in blood. However, incorporation of multiple dyes into a single sensor complicates the manufacture of such sensors.
Besides the foregoing problems associated with separate indwelling sensors for each analyte being monitored, particularly in the intensive care setting, and multiple dye sensors, another problem associated with many dye-based analyte sensors is pH sensitivity. A slight change in pH may modify or attenuate indicator emissions, and cause inaccurate readings. This problem is particularly acute for monitoring blood glucose levels in diabetic and non-diabetic ICU patients, whose blood pH may fluctuate rapidly. Since accurate blood glucose level measurements are essential for treating these patients, there is a significant need for a glucose sensor that facilitates real-time correction of the pH effect without requiring separate indwelling pH and analyte sensors, or sensors having multiple dyes. Accordingly, in order for acutely ill patients in the ICU setting to enjoy the benefit of tight glycemic control, there remains an important unmet need for a glucose sensor configured for intravascular deployment, wherein the sensor employs a non-consuming, equilibrium chemistry that provides accurate, real-time glucose levels which simultaneously monitors and corrects for fluctuations in blood pH.